A flexible future

The solar power industry is facing great change with the advent of thin-film technology but, as E&T discovers, there are still several paths for the sector to choose from.

By 2020, solar cells will be everywhere - and you may not even be able to see them. They may be transparent, integrated into your office window. They may form a thin film on the roof of your car. Solar cells may be integrated into your briefcase or rucksack, charging up your mobile devices while you are outside and even while you are in your office. They may even be on the top of your television, reusing the light from your living room bulb, enabling you to leave your TV on standby all night.

These applications will be made possible because the solar cells of tomorrow are not to made using the same technology as today. Today’s solar cells, which are made using silicon or other rare - often toxic - semiconductors, are complicated to manufacture, and can be cumbersome. Thin, flexible solar cells, will be made using either polymers, a photosensitive dye or organic small-molecule technology and promise cheap, portable solar power (see ‘technology backgrounder’ p44). They will be much simpler to manufacture and also will not use toxic, expensive materials.

After more than 20 years of research and development these cells are today coming to market and by 2020 are predicted to be a prevalent technology. Effectively the reverse of an LED - converting light into electricity rather than electricity into light - thin-film photovoltaic cells have benefited hugely from the immense research in organic LEDs (OLEDs). The materials and manufacturing technology for both OLEDs and thin-film photovoltaics is remarkably similar.

But these solar cells also suffer from the same problems that OLEDs had at the start. The biggest issues are lifetime and efficiency.

Solar cell efficiency

Today’s most efficient solar cells have an efficiency of around 15-20 per cent during the hottest part of the day. While dye-sensitised and organic cells cannot yet match this efficiency, great strides have been made in recent years. Just two years ago, the best efficiencies reported were around 10 per cent and 4 per cent respectively. Today, these figures are closer to 12 per cent and 8 per cent respectively, with the most recent announcement coming from

US company Solarmer, a manufacturer of polymer cells. The company announced in December last year that it had achieved an efficiency of 7.9 per cent.

Lifetimes are also increasing, with companies estimating lifetimes of up to 15 years for their products. While these figures look promising, it must be pointed out that these are for small cells performing under laboratory conditions, and the cell that gives the highest efficiency will not be the one that gave the longest lifetime. Once the manufacture of these cells has been scaled up - and the cells have been both encapsulated to protect them against the elements and integrated into commercially viable modules - their efficiency is reduced by 50 per cent or more. In reality, the best performance that can be expected from a commercially-available module - as opposed to a cell - is currently less than 4 per cent and closer to 1 per cent.

However, while flexible solar cells may not be as efficient as current technology, they do have many other advantages.

“Our polymer cells are affordable, flexible and able to perform in low or variable lighting conditions,” says Stuart Spitzer, vice president of engineering at US company, Konarka. “Silicon cells may be very efficient, but their efficiency drops off as the sun moves across the sky. The efficiency of polymer solar cells is more constant throughout the day and the cells can pick up the sun’s energy from lower angles.”

It is for this reason that polymer cells can be used in vertical surfaces such as walls and even windows. Konarka recently installed a solar curtain wall at Florida company Arch Aluminium & Glass. When fully commissioned, this curtain wall is expected to produce 1.5kW of power. “The purpose of this project is to test the performance and robustness of our solar panel solution for a curtain wall application,” says Spitzer. “Before we installed the panels, the concrete vertical wall was not generating any power at

all, so even with our lower efficiency - which is better than no power at all - this wall will enable us to show developers and architects what our technology can do and will also enable us to test out our lifetimes and other performance data on a real application.”

Konarka makes its cells using a modified printing process, and is therefore able to produce large areas of cells very quickly. “We can make cells up to 150cm-wide at 0.5m/s and we plan to run faster in the future,” says Spitzer. The process runs at room temperature and does not need a clean room, unlike today’s silicon cell manufacturing process. “Due to available equipment capacity in the printing industry, capital costs of polymer photovoltaics production line are between 5 per cent and 10 per cent that of a silicon production line,” says Spitzer.

Mobile applications

As well as building-integrated applications, Spitzer sees many mobile applications for his company’s products, including applications in developing countries where small amounts of electricity are needed after dark to power lights, for example. “A sheet of cells about the size of an A4 piece of paper can charge two AA batteries in two hours,” he says. “This could be used to power a light to enable a child to do his or her school work, for example.”

While Konarka has made great strides in commercialising polymer solar cells, it started out developing dye-sensitised solar cells, a flexible technology that works on a different principle to polymer cells. “In 2004, we decided to move to polymer technology for several reasons,” says Spitzer. “The main reason was the liquid electrolyte. It was a challenging material to work with. We had a great efficiency, but I would speculate that once companies such as Konarka put in the same man years into polymer technology that have been invested into dye-sensitised technology, we can exceed the performance of dye-sensitised cells.”

That may be the case, but currently, dye-sensitised technology is several strides ahead of polymer technology - both in performance and commercial products. Konarka sold its manufacturing technology to UK company G24 Innovations in 2004, and at the end of last year, G24 announced the first ever commercial shipment of dye-sensitised photovoltaic modules to Hong Kong-based consumer electronics bag manufacturer, Mascotte Industrial Associates.

“We deliver 150 × 200mm modules to Mascotte and the company integrates them into the bags,” explains Roy Bedlow, chief marketing officer at G24i. “The modules deliver 0.6W of power, which is enough to extend the talk-time of a mobile phone, or recharge a mobile device in a day.”

The lifetime of G24i’s products matches those of the intended application. So for a consumer product, this lifetime might be in the region of five years.

Like most manufacturers of flexible solar cells, Bedlow admits that G24i’s technology is not competing with grid applications, but rather that the technology can reduce the power draw on the grid. “8 per cent of the average UK electricity bill is due to standby power,” says Bedlow. “Our technology cannot power a TV, but it can certainly power a device in standby mode. We cannot change consumer habits, but if all consumer devices had dye-sensitised solar cell technology integrated into them, we can save the UK 8 per cent of its energy bills by powering the standby mode on devices using energy from indoor lighting during the day. That’s a considerable saving.”

Applications such as these have captured the imagination of the investment community and start-up companies developing flexible solar cells have sprung up all over the world. German company Heliatek recently received $27m in a second round of financing from a group of investors that include electronics giant Bosch and chemicals company BASF.

Heliatek’s technology is fundamentally different to dye-sensitised and polymer technology in that it uses vacuum deposition as a manufacturing process. “Just as small-molecule technology has won the race in the OLED industry, we believe that small-molecule organic solar cell technology will be the technology of choice in the future,” says Andreas Rückemann, CEO of Heliatek. “While printing solar cells is an intriguing technology, we believe these companies will have problems in the future.”

Heliatek has achieved efficiencies greater than 6 per cent by employing a tandem technology - stacking cells on top of each other in order to cover a large range of the solar spectrum. “We believe tandem technology is the way forward, and this is very difficult to do when printing cells without dissolving the bottom layer,” says Rückemann.

Heliatek has reported 4.4 per cent efficiency for a module - the largest reported efficiency for any organic technology.

The effective area efficiency of that module is 5.3 per cent, only 10 per cent different to the efficiencies reported for its cells.

When it comes to talking about applications, Rückemann is more cautious than other proponents of organic photovoltaic technology.

“We are really pleased that companies are getting flexible solar cells into consumer applications, as this raises awareness of the technology,” he told E&T magazine. “But we are aiming at the large-area applications and may even, by 2020, have some grid applications, something we did not think possible when we started work on this technology. Whatever applications prevail, they must be financially viable, and this must be calculated very carefully.”